This is a multicenter clinical trial to compare the safety and efficacy of 40 mg f sorivudine administered once daily for 7 days to 800 mg acyclovir given every four hours five times a day for 7 days in the treatment of acute localized zoster in immunocompetent adults. Three hundred forty-eight immunocompetent adults, who are greater than 50 years of age will be enrolled. Adults greater than 18 years of age with trigeminal involvement will also be allowed to enroll. Subjects are enrolled within 72 hours of the appearance of zoster rash. Zoster is confirmed by direct fluorescent antibody analysis. The duration, incidence and intensity of zoster-associated pain, the effects on healing of skin lesions, on quality of life and zoster-related health care costs in the two treatment groups will be assessed over a six month period. The enrollment and treatment stages of this trial have been completed with no serious adverse effects reported.